Downing Street says it stands by its record of protecting the environment and delivering a low carbon economy.

A year ago, David Cameron said the environment would be a top priority.

In their letter the campaign groups describe the pledge as a "great ambition" and cite the cancellation of the third runway at Heathrow, the decision to set up a green investment bank and a commitment to a Natural Environment White Paper as examples of a "promising start".

They say the coalition "started with a strong sense of purpose on the environment but is now in danger of losing its way".

We urge you to set out the case that a green economy is central to the future prosperity of the UKCampaign groups

They point to the proposed changes in the planning system which they claim will not provide enough protection for wildlife and the countryside.

The letter also suggests a zero carbon homes policy has been weakened and point to delays in giving borrowing powers to the new green investment bank.

It says foreign policy should take account of natural resource security.

The letter adds: "Most critically we urge you to set out the case that a green economy is central to the future prosperity of the UK and not a cost to be endlessly debated and watered down...

"We believe there is still scope for your government to be the greenest ever, but it will require both urgency and resolve."

The letter is also signed by the heads of Green Alliance, Christian Aid, WWF, the Campaign to Protect Rural England, the Campaign for Better Transport, Oxfam, the Institute for European Environmental Policy, the Wildlife Trusts, Butterfly Conservation, Plantlife, the Wildfowl and Wetlands Trust and the Woodland Trust.

60% cuts

It goes on to urge the prime minister to accept the key recommendations of advisory body the Committee on Climate Change on cutting emissions "to create clarity for investors and citizens about the UK's direction".

Andy Atkins, executive director of Friends of the Earth, told the BBC's Today programme that if the government failed to accept the committee's key recommendations, it would make it "very difficult" for it "to achieve many of the other things that it says it wants to do".

He added: "We think that out of integrity they've really got to do it if they want to be seen as the greenest government ever."

Mr Atkins said he was concerned that the government was moving away from accepting the committee's recommendations because of economic considerations, which he called "stone age economics".

"Let's be clear, the climate change committee has not just made these recommendations on the basis of the science, they have looked at the economics as well," he added.

Cabinet split

The campaigners' plea comes after reports of a split in the cabinet over whether to accept the recommendations amid concerns that meeting the goal could impact on economic growth.

Labour has seized on the apparent disagreement, saying it left Mr Cameron's environmental commitment "in shreds".

Last week, a review funded by Friends of the Earth suggested there had been little or no progress in 55 government policies on the environment.

In response a government spokesman highlighted initiatives such as the "green deal" to cut home emissions, its work on global deals to protect wildlife and forests and reforms of the electricity market to promote low carbon generation.